How do I make a USB installer with Catalina?

Have you tried this,

How to upgrade to macOS Catalina – Apple Support

Go to Section 4 and click on Get macOS Catalina

this should redirect you to the App Store Catalina download page.

It should say Get or Download.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS Catalina.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

With Install macOS Catalina.app in the Applications folder you can then make

a bootable USB installer with these instructions,

How to create a bootable installer for macOS – Apple Support

Apple still supports creating bootable USB installers.

How to create a bootable installer for macOS – Apple Support

If your mac is on 10.15.6 then it should be fully up to date.

Open System Preferences> Software Update and check to see if it detects anything.

And are you using a cabled connection to your router.

I really do not understand why you cannot download the Install macOS Catalina.app,

but my suspicions are that it is downloading, and you are not catching it before the

Installer starts up. The installer is noticing you have already got Catalina installed, quits

then deletes Install macOS Catalina.app

Each time you have attempted to do this have you restarted your mac so caches are cleared

out.

It might also be worth trying this in Safe Mode.

How to use safe mode on your Mac – Apple Support

The second picture shows it downloading with 19 minutes to go, did it get any further, at what point is the error coming up.

The third picture shows that an error has occurred while trying to install, sometimes having download the full installer the

installation process will start up automatically. This is what may have happened and the installer is going hang on Catalina already exist on this mac and refuses to go any further.

Are you sure it hasn't fully downloaded, look in the Applications folder for Install macOS Catalina.app if it has fully downloaded

it should be 8.24GBs in size.

If you are trying to download from Apples Servers it is often recommended to be connected to your router via cable and not WiFi.

Do not use the mac while it is downloading and do not use any other devices that share the same internet connection.

And then what happens, does it not begin the download of Catalina, give it a go.

Or does it say Catalina is already installed and then refuses download it.

Any chance you can share some screenshots.

Very good.